1. Field of the Invention
The present invention relates to the general field of holographic storage systems and methods. More specifically the invention relates to a system and method for a bitwise readout of a holographic ROM that may be backward compatible to existing or future storage formats such as DVD or CD-ROM.
2. Description of Related Art
General holographic storage systems are discussed in xe2x80x9cHolographic Memories,xe2x80x9d by Demetri Psaltis et al., Scientific American, November 1995, which is hereby incorporated by reference. Holography is also discussed in the text Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag which is hereby incorporated by reference. The basic principles of holography involve the recording of an interference pattern formed between two beams of light, referred to as an object beam and a reference beam. The object beam is encoded with data in a two dimensional pattern. The reference beam is used to form the interference pattern with the encoded object beam and is subsequently used to reconstruct the data by illuminating the recorded pattern.
In volume holographic storage, a large number of holograms are stored in the same volume region of a holographic storage medium. There are several method of holographic storage, such as shift multiplexing, angle multiplexing, wavelength multiplexing, correlation multiplexing and phase multiplexing. Volume holography uses a thick recording medium, where the thickness dimension is associated with Bragg selectivity in the movement of the holographic storage medium in shift multiplexing or the angle change in angle multiplexing.
A prior art holographic system is described in xe2x80x9cHolographic 3-D Disc using In-line Face-to-Face Recording,xe2x80x9d by Kimihiro Saito and Hideyoshi Horimai. The system described utilizes a photosensitive layer with a reflecting unit underneath. A reference beam passes through a first region of the media downward and a second region upwards. The direction of the information beam is opposite to that of the reference beam. Intersection between the reference beam and information beam results in a reflection type hologram. Shift multiplexing can be utilized for multiple recording.
Angle multiplexing is a volume holography method for storing a plurality of images within a single storage medium. Such angle multiplexing is discussed, for example, in xe2x80x9cHolographic Memories,xe2x80x9d by Demetri Psaltis et al., Scientific American, November 1995, and by P. J. van Heerden in, xe2x80x9cTheory of Optical Information Storage In Solids,xe2x80x9d Applied Optics, Vol. 2, No. 4, page 393 (1963). A typical system employing angle multiplexing described in xe2x80x9cHolographic Data Storage,xe2x80x9d pages 343-397, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. Angle multiplexing generally involves storage of multiple pages of data in the same photorecording medium by altering the angle of the reference beam entering the media during storage of each page while maintaining the position of the object beam. Each hologram is stored in the same volume and is differentiated by Bragg selectivity. Bragg selectivity during angle multiplexing is described in Holographic Data Storage, pages 30-38, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. Any of the recorded holograms can be viewed by illuminating the photorecording medium with a reference beam set at the appropriate angle.
Wavelength multiplexing is a further method for storing a plurality of images within a single medium, whereby the addressing mechanism is the wavelength of incidence of the reference beam. Wavelength multiplexing is simpler to implement than angle multiplexing, but it is highly dependent on the range over which lasers can be tuned. Wavelength multiplexing is described in xe2x80x9cHolographic Data Storage,xe2x80x9d pages 7-8 and 25-26, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag.
A prior art system geometry in which the encoded object beam and the recording reference beam are counterpropagating is described in xe2x80x9cVolume Holographic Multiplexing Methodsxe2x80x9d, by G. Barbastathis and D. Psaltis, published in Holographic Data Storage, pages 22-59, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag, which is expressly incorporated herein by reference. This geometry is often preferred in wavelength multiplexed systems because it maximizes the optical wavelength Bragg selectivity.
Compact discs (CDs) and digital video disks (DVDs) are currently popular optical recording formats. Both CD and DVD players are readily available at low cost. FIG. 5 illustrates the basic setup of a typical read system utilized to recall data stored on an optical media such as a CD or DVD. The read system 500 includes a laser light source 502, which provides a plane wave readout beam 503. The readout beam from the laser light source 502 travels through a diffraction grating 504, a collimator 508, a beam splitter 506, quarter waveplate 510, and objective lens 512. Objective lens 512 focuses the readout beam onto a reflective layer 514 that is the bottom layer of the CD or DVD containing data. Readout beam 503 is a spherical beam passing through objective lens 512. Readout beam 503 is reflected or not reflected depending on the data, with the reflected readout beam 503 a spherical wave off the reflective layer 514.
The readout beam is then reflected back through objective lens 512, quarter wave plate 510, and to the beam splitter 506. A plane wave is produced after the reflected spherical beam retro-reflects through objective lens 512. The readout beam is reflected towards lens 516 and is imaged onto the plane of an optical detector 520, which is typically a quad detector. Laser light source 510 is generally a low power (5-10 mW), relatively inexpensive laser with multiple longitudinal mode emission. Collimator lens 508 is placed a distance away from the laser, on the opposite side of the beam splitter 506 from the laser. Readout beam 503 diverges rapidly, resulting in the collimator lens 508 illuminated by the central core of readout beam 503. In addition to beam splitter 506, a beam-turning element may be placed between laser 502 and collimator lens 508 in order to flatten the system profile for use in a compact drive. After the readout beam 503 passes through collimator lens 508, it is well collimated and can be focussed by objective lens 512. The reflected readout beam is recollimated by objective lens 512 and proceeds back through the collimator lens 508, which acts as a field lens. The reflected readout beam is deflected by the beam splitter and focussed on the detector, which senses the high-frequency data signal as well as tracking and focus error signals. It should be recognized that FIG. 5 is illustrative only. Current read systems or optical pick-up systems typically combine various elements to reduce the number of elements and cost of the system.
Additionally, the current use of holography in commercial ROM systems and storage media is on the backside (i.e., the side opposite the reflective layer) of the CD or DVD. Here single holograms of pictures and words (e.g., names or titles) are sometimes recorded for security or authenticity reasons.
Although prior art DVD and CD players are readily available, low cost players that can also read holographic ROM discs are not available. Furthermore, ROM disc replication is done by stamping the information onto the surface of the disk. Thus, there has been a need for improvements in the recording (replication of the information efficiently and quickly) and readout of holograms. More specifically, there has been a need for improved systems capable of reading holograms and other optical media.
According to one example of one aspect of the present invention a method is provided for manufacturing a holographic storage medium. The method includes providing one or more data masks with data to be recorded on a holographic storage medium, illuminating the one or more data masks onto the holographic storage medium with a plane wave object beam from a laser light source operating at a record wavelength, propagating a reference beam at an incident angle to the holographic storage medium to record the one or more data masks on the holographic storage medium, and altering the incident angle of the reference beam for each of the one or more data masks, wherein each of the one or more data masks recorded on said holographic storage medium can be read bit by bit using a laser light source operating in a readout range of wavelengths different from the record wavelength.
According to another example of an aspect of the present invention a method is provided for manufacturing a holographic storage medium including providing one or more data masks with data to be recorded on a holographic storage medium, propagating a plane wave object beam and reference beam from a laser light source operating at a first range of record wavelengths, illuminating the one or more data masks onto the holographic storage medium with the object beam, propagating the reference beam at fixed incident angle to the holographic storage media to record the one or more data masks on the holographic storage media, and altering the record wavelength of the object beam and reference beam for each of the one or more data masks, wherein each of the one or more data masks recorded on said holographic storage medium can be read bit by bit using a laser light source operating in a readout range of angles different from the first range of record angles.
The present invention is better understood upon consideration of the detailed description below in conjunction with the accompanying drawings and claims where various other examples and aspects of the present invention are described in greater detail.